Currently most civilian positioning receivers are single-band Global Positioning System (GPS) receivers. These receivers use the L1-CA code broadcast by GPS satellites at a frequency 1575.42 MHz, as defined for GPS protocols. The performance of these receivers is limited by the signal strength, chip rate, data rate, code length and availability of the L1-CA signal. Due to the modernization plan of the United States GPS system and the planned deployment of the European Galileo system, more satellite signals will be available in the near future for civilian use. These signals will dramatically improve the capability and performance of positioning receivers by offering higher code rates and data rates, longer codes, higher power, frequency diversity, and increased availability.
The GPS modernization plan includes introducing two additional satellite signals for civilian use: L2 at 1227.6 MHz and L5 at 1176.45 MHz. The European Galileo system will offer thirty new satellites, with positioning signals in four frequency bands: E5a at 1176.45 MHz, E5b at 1207.14 MHz, E6 at 1278.75 MHz and E2 at 1575.42 MHz. Although some of the Galileo bands coincide with the GPS bands, they are designed to not interfere with the GPS signal codes.
To take full advantage of these additional signals, future positioning receivers will need to operate in multiple frequency bands. The problem is that each additional band will require additional hardware with additional cost, which is prohibitive for many low-cost civilian applications. Also, since it is not practical to implement all the bands, a receiver developer must decide which bands to include in the receiver, without any test data of how these new signals will perform when deployed. The problem is how to design a dynamically reconfigurable multi-band receiver, with minimum hardware and cost.
Several multi-band architectures and methods have been proposed for dual band receivers using L1 and L2 GPS frequencies. U.S. Pat. No. 5,736,961 uses different fixed-frequency downconverters for the L1 and L2 signal with complete duplication of the downconversion hardware. This is not a practical dual-band solution, and not scalable to more frequencies. U.S. Pat. No. 5,040,240 also uses separate receiver chains for L1 and L2 frequencies, but shares a common frequency synthesizer. Therefore, this technique also suffers from duplication of hardware. The receiver disclosed in U.S. Pat. No. 6,675,003 separates the L1 and L2 signals at a second image reject mixer. This is an efficient solution for a L1/L2 dual-band receiver but is not scalable to more frequencies without significant increase in hardware.
U.S. Pat. No. 6,081,691 discloses a GPS/GLONASS (Global Orbiting Navigation Satellite System) receiver capable of receiving satellite signals from a single-frequency GPS system and a multiple-frequency GLONASS system, and WIPO Patent Application WO 01/39364 presents another method of implementing a multi-band GPS/GLONASS receiver. For both these references, the IF processing paths are fixed and cannot be electronically reconfigured for different frequencies. Moreover, the GLONASS system, which has been in decline since 1996 has a limited lifetime.
There is other prior art that covers multi-band receivers in general, used for any application. For example, U.S. Patent Application 2002/0173337 A1 and U.S. Pat. No. 6,088,348 disclose dual-band or tri-band architectures for cellular, PCS and GPS frequencies. However, these architectures use separate mixers for the first down-conversion, and switched PLL synthesizers, which adds additional hardware cost and complexity.
What is needed is an improved electronically reconfigurable downconverter for a multi-band positioning receiver. It would also be of benefit to provide such improvement with a receiver having a minimum of additional hardware requirements